1. Field of the Invention
The invention relates to a device for optical heterodyne detection of an optical signal beam. The device comprises a local oscillator, means for combining local oscillator radiation with signal beam radiation, an opto-electric converter and a plurality of controllable elements to influence the state of polarisation, which elements have a limited control range.
The invention also relates to an optical transmission system provided with such a device.
2. The Prior Art
As compared with direct detection of a signal beam, heterodyne detection provides considerable advantages relating to the signal-to-noise ratio and the discrimination from background radiation. The principle of heterodyne detection for optical radiation has been described extensively in the Article "Optical Heterodyne Detection" by O.E. DeLange in the Journal "IEEE Spectrum" of October 1968, pages 77-85. As stated in this article it is important that the states of polarisation of the modulated signal beam and of the local oscillator beam correspond as much as possible. To achieve this, polarisation control is necessary, because optical transmission systems use optical waveguides or optical fibres, which disturb the state of polarisation of the radiation propagating therein. These fibres, which may be several dozen to several hundred kilometres long, are subjected to uncontrollable external influences such as temperature and pressure variations so that the disturbance of the state of polarisation may vary on time. A signal beam linearly polarised at the input of a transmission fibre will generally be distorted at the output to an elliptically polarised beam whose ellipticity and direction of major axis vary over time.
The Article "Method of overcoming finite-range limitation of certain state of polarisation control devices in automatic polarisation control schemes" by L. J. Rysdale in the Journal "Electronics Letters", Vol. 22, No. 2, 16th Jan. 1986, pages 100-102, proposes a method using four controllable elements to influence the state of polarisation. The elements have a limited range and are arranged one after the other. This leads to a system with which the linearly polarised local oscillator beam is brought to a state of polarisation which corresponds to the state of polarisation of the signal beam. With this system changes in the state of polarization of the signal beam can be followed to an unlimited extent.